[PATCH 5.10 071/406] refscale: Fix uninitalized use of wait_queue_head_t

Greg Kroah-Hartman
2023-09-17T19:22:34Z

From: Waiman Long <longman@redhat.com>

[ Upstream commit f5063e8948dad7f31adb007284a5d5038ae31bb8 ]

Running the refscale test occasionally crashes the kernel with the
following error:

[ 8569.952896] BUG: unable to handle page fault for address: ffffffffffffffe8
[ 8569.952900] #PF: supervisor read access in kernel mode
[ 8569.952902] #PF: error_code(0x0000) - not-present page
[ 8569.952904] PGD c4b048067 P4D c4b049067 PUD c4b04b067 PMD 0
[ 8569.952910] Oops: 0000 [#1] PREEMPT_RT SMP NOPTI
[ 8569.952916] Hardware name: Dell Inc. PowerEdge R750/0WMWCR, BIOS 1.2.4 05/28/2021
[ 8569.952917] RIP: 0010:prepare_to_wait_event+0x101/0x190
:
[ 8569.952940] Call Trace:
[ 8569.952941] <TASK>
[ 8569.952944] ref_scale_reader+0x380/0x4a0 [refscale]
[ 8569.952959] kthread+0x10e/0x130
[ 8569.952966] ret_from_fork+0x1f/0x30
[ 8569.952973] </TASK>

The likely cause is that init_waitqueue_head() is called after the call to
the torture_create_kthread() function that creates the ref_scale_reader
kthread. Although this init_waitqueue_head() call will very likely
complete before this kthread is created and starts running, it is
possible that the calling kthread will be delayed between the calls to
torture_create_kthread() and init_waitqueue_head(). In this case, the
new kthread will use the waitqueue head before it is properly initialized,
which is not good for the kernel's health and well-being.

The above crash happened here:

static inline void __add_wait_queue(...)
{
:
if (!(wq->flags & WQ_FLAG_PRIORITY)) <=== Crash here

The offset of flags from list_head entry in wait_queue_entry is
-0x18. If reader_tasks[i].wq.head.next is NULL as allocated reader_task
structure is zero initialized, the instruction will try to access address
0xffffffffffffffe8, which is exactly the fault address listed above.

This commit therefore invokes init_waitqueue_head() before creating
the kthread.

Fixes: 653ed64b01dc ("refperf: Add a test to measure performance of read-side synchronization")
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Qiuxu Zhuo <qiuxu.zhuo@intel.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/rcu/refscale.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/rcu/refscale.c b/kernel/rcu/refscale.c
index 4e419ca6d6114..dbd670376c42e 100644
--- a/kernel/rcu/refscale.c
+++ b/kernel/rcu/refscale.c
@@ -692,12 +692,11 @@ ref_scale_init(void)
VERBOSE_SCALEOUT("Starting %d reader threads\n", nreaders);

for (i = 0; i < nreaders; i++) {
+ init_waitqueue_head(&reader_tasks[i].wq);
firsterr = torture_create_kthread(ref_scale_reader, (void *)i,
reader_tasks[i].task);
if (firsterr)
goto unwind;
-
- init_waitqueue_head(&(reader_tasks[i].wq));
}

// Main Task
--
2.40.1